This disclosure pertains to microlithography, which involves projection-transfer of a pattern, defined by a reticle or mask, to a xe2x80x9csensitivexe2x80x9d substrate using an energy beam. By xe2x80x9csensitivexe2x80x9d is meant that the substrate is imprintable with an image carried from the reticle to the substrate by the energy beam. More specifically, this disclosure pertains to reticles as used in microlithography in which the energy beam is an electron beam.
As noted above, in projection microlithography the pattern is defined on a reticle or mask (termed a xe2x80x9creticlexe2x80x9d herein) and is projected onto the sensitive substrate. For electron-beam (EB) microlithography, various types of reticles have been devised, including xe2x80x9cabsorptionxe2x80x9d reticles and several types of xe2x80x9cscatteringxe2x80x9d reticles.
Scattering reticles generally are of two types: xe2x80x9cscattering-stencilxe2x80x9d reticles and xe2x80x9cscattering-membranexe2x80x9d reticles. A scattering-stencil reticle comprises an electron-beam-scattering (EB-scattering) membrane, usually made of silicon (Si), that defines pattern elements by corresponding EB-transmissive through-holes in the membrane. A scattering-membrane reticle comprises a thin EB-transmissive membrane, usually made of silicon nitride (SiN), on which pattern elements are defined by corresponding portions (xe2x80x9cscattering bodiesxe2x80x9d) of a thin EB-scattering layer (e.g., tungsten (W) or chromium (Cr)) on the SiN membrane. An xe2x80x9cabsorptionxe2x80x9d reticle is constructed similarly to the scattering-membrane reticle, except that pattern elements are defined by corresponding xe2x80x9cabsorption bodiesxe2x80x9d of a EB-absorbing material (e.g., heavy metal) on a EB-transmissive membrane.
A scattering reticle is used in a different manner than an absorption reticle. With an absorption reticle, portions of the incident beam are absorbed by the EB-absorbing bodies, which can cause localized heating of the reticle. With a scattering reticle, in contrast, the incident electron beam is scattered (mostly forward-scattered) as it passes through various portions of the reticle, rather than being absorbed by the reticle. The resulting scattering contrast defines pattern elements as imaged on the substrate. Since few of the incident electrons are absorbed, the temperature of a scattering reticle can be kept relatively low compared to an absorption reticle, with a corresponding increase in the accuracy of pattern transfer.
With a scattering-stencil reticle, the incident electrons that pass through the through-holes of the reticle (and that do not interact with the reticle and become scattered, for example) are used for transfer and exposure. Hence, the full capacity of the EB-optical system can be used, with a corresponding increase in imaging resolution. A disadvantage of scattering-stencil reticles, however, is the fact that certain pattern elements (e.g., donut-shaped elements, peninsular-shaped elements, certain longitudinally extended elements) cannot be defined entirely in a single corresponding portion of the reticle. Rather, complete definition of such a pattern element on the reticle requires use of complementary portions of the reticle each defining a respective portion of the element. As exposed onto the substrate, the images of the complementary pattern portions must be xe2x80x9cstitchedxe2x80x9d together properly to create the entire respective pattern element. The unusually high stitching accuracy required poses a problem with using this type of reticle.
Scattering-membrane reticles also have problems, notably the fact that some of the energy of the incident electrons passing through the thin EB-transmissive membrane is absorbed by the scattering bodies. This energy absorption not only increases the temperature of the reticle but also results in increased chromatic aberration of the electron beam propagating downstream of the reticle. I.e., the chromatic aberration arises from energy of the incident beam being absorbed by certain regions of the reticle relative to other regions.
In view of the foregoing, an object of the invention is to provide, inter alia, scattering-reticle assemblies for use in electron-beam (EB) microlithography of a pattern, defined by the reticle assembly, to a substrate. An embodiment of such a reticle assembly comprises at least one scattering-stencil reticle portion defining a respective portion of the pattern, and at least one scattering-membrane reticle portion defining a respective portion of the pattern. Typically, the reticle assembly is of a xe2x80x9cdividedxe2x80x9d reticle, wherein each of the scattering-stencil reticle and scattering-membrane reticle portions is segmented into respective subfields. Also, the reticle assembly desirably further comprises a support frame, wherein the at least one scattering-stencil reticle portion and the at least one scattering-membrane reticle portion are bonded to respective locations on the support frame.
Each scattering-stencil reticle portion typically comprises a respective reticle membrane made of silicon, wherein pattern elements are defined by respective through-holes in the reticle membrane. Each scattering-membrane reticle portion typically comprises a respective reticle membrane made of Si, SiC, SiN, carbon, diamond, BN, or a mixture thereof. Each scattering-membrane reticle portion typically further comprises a respective pattern-defining layer on the respective reticle membrane. The pattern-defining layer desirably is made of a material such as chromium (Cr), tungsten (W), tantalum (Ta), or a mixture thereof. Further desirably, each respective pattern-defining layer has a transmittance to electrons of an incident electron beam of no more than one-tenth of the respective reticle membrane.
The reticle assembly can comprise a first scattering-stencil reticle portion, a second scattering-stencil reticle portion, and a scattering-membrane reticle portion. In such a configuration, the second scattering-stencil reticle portion usually comprises subfields defining respective pattern features that are complementary to respective pattern features defined in subfields of the first scattering-stencil reticle portion.
Also provided are EB microlithography apparatus. An embodiment of such an apparatus includes an illumination system, a projection system, and a scattering-reticle assembly. The illumination system is situated and configured to receive an illumination beam from an EB source and to direct the illumination beam to a selected region on the reticle assembly. The projection system is situated and configured to direct a portion of the beam passing through the reticle assembly to a substrate. The scattering-reticle assembly is situated on a reticle stage so as to be illuminated by the illumination beam. The scattering-reticle assembly comprises at least one scattering-stencil reticle portion defining a respective portion of the pattern, and at least one scattering-membrane reticle portion defining a respective portion of the pattern. Each of the scattering-stencil reticle and scattering-membrane reticle portions of the reticle assembly can be configured as summarized above with respect to reticle assemblies.
Also provided are improvements to EB microlithographic exposure methods in which an electron beam is directed to a reticle assembly (that defines a pattern) by which the beam acquires an ability to form an image of the pattern on a substrate. On the reticle assembly a first pattern portion is defined on at least one scattering-stencil reticle portion, and a second pattern portion is defined on at least one scattering-membrane reticle portion. Using the electron beam, the pattern portions from each reticle portion are transferred to the substrate so as to produce a stitched-together image of the pattern on the substrate.
Characteristic of a divided reticle assembly, each of the reticle portions typically is divided into respective subfields each defining a respective portion of the pattern. With such a reticle assembly, the step of transferring the pattern portions comprises exposing the subfields individually to respective locations on the substrate so as to stitch together images of the respective portions of the pattern on the substrate. Also, each of the scattering-stencil reticle and scattering-membrane reticle portions of the reticle assembly can be configured as summarized above with respect to reticle assemblies. In general, each scattering-stencil reticle portion comprises a respective reticle membrane defining pattern elements by respective through-holes in the reticle membrane, and each scattering-membrane reticle portion comprises a respective reticle membrane including a pattern-defining layer made of a material different from a material used to form the respective reticle membrane.
Also provided are apparatus for performing a fabrication process on a wafer as required to produce a microelectronic device on the wafer. An embodiment of such an apparatus comprises an EB optical system and a scattering-reticle assembly situated relative to the EB optical system and defining a pattern that is projection-transferred to a substrate by an energy beam passing through the EB optical system. The scattering-reticle assembly comprises at least one scattering-stencil reticle portion defining a respective portion of the pattern, and at least one scattering-membrane reticle portion defining a respective portion of the pattern. The scattering-reticle assembly can further comprise a support frame, wherein the at least one scattering-stencil reticle portion and the at least one scattering-membrane reticle portion are bonded to respective locations on the support frame. Each of the scattering-stencil reticle and scattering-membrane reticle portions of the reticle assembly can be configured as summarized above with respect to reticle assemblies.
Also provided are methods for fabricating a reticle assembly defining a pattern to be transferred microlithographically to a substrate using an electron beam. In an embodiment of such a method, a scattering-stencil reticle portion is fabricated that defines a first portion of the pattern. Also, a scattering-membrane reticle portion is fabricated that defines a second portion of the pattern. The scattering-stencil reticle portion and the scattering-membrane reticle portion are attached to a reticle frame. Each of the scattering-stencil reticle and scattering-membrane reticle portions of the reticle assembly can be configured as summarized above with respect to reticle assemblies.
With any of the reticle assemblies summarized above, the need to stitch together complementary pattern portions on the reticle is not increased over the need experienced with conventional reticles. In fact, the need for such stitching usually is reduced compared to what is encountered with conventional scattering-stencil reticles. In addition, reticle assemblies as summarized above do not produce increased chromatic aberrations. Hence, by including at least one of each type of scattering reticle on a single reticle assembly, it is now possible to compensate for the shortcomings of the two types of scattering reticles.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.